Free Astronomy Magazine July-August 2019

53 JULY-AUGUST 2019 SPACE CHRONICLES T his graphic re- veals close-up images of 15 galax- ies from the 265,000 galaxies in the Hubble Legacy Field. The galaxies are scattered across time, from 550 mil- lion years ago to 13 billion years ago. Their light is just arriving at Earth now, after crossing space for all those years. This collec- tion of images al- lows astronomers to look back in time to see galax- ies when they were very young, in the earliest epochs of the universe. The universe is 13.8 billion years old. The top panel of snapshots shows mature "adult" galaxies; the middle panel shows galaxies in their "teenage" years when they are growing and changing dramatically; and the bottom panel shows small, youthful galaxies. [NASA, ESA, G. Illingworth and D. Magee (University of California, Santa Cruz), K. Whitaker (University of Connecticut), R. Bouwens (Leiden University), P. Oesch (University of Geneva), and the Hubble Legacy Field team] sions, is located at the Space Tele- scope Science Institute in Baltimore, Maryland. The Hubble Space Telescope has come a long way in taking ever deeper “core samples” of the dis- tant universe. After Hubble’s launch in 1990, astronomers debated if it was worth spending a chunk of the telescope’s time to go on a “fishing expedition” to take a very long ex- posure of a small, seemingly blank piece of sky. The resulting Hubble Deep Field image in 1995 captured several thousand unseen galaxies in one pointing. The bold effort was a landmark demonstration and a defining proof- of-concept that set the stage for fu- ture deep field images. In 2002, Hubble’s Advanced Camera for Sur- veys went even deeper to uncover 10,000 galaxies in a single snapshot. Astronomers used exposures taken by Hubble’s Wide Field Camera 3 (WFC3), installed in 2009, to assem- ble the eXtreme Deep Field snapshot in 2012. Unlike previous Hubble cam- eras, the telescope’s WFC3 covers a broader wavelength range, from ul- traviolet to near-infrared. This new image mosaic is the first in a series of Hubble Legacy Field im- ages. The team is working on a sec- ond set of images, totaling more than 5,200 Hubble exposures, in an- other area of the sky. In the future, astronomers hope to broaden the multiwavelength range in the legacy images to include longer- wavelength infrared data and high- energy X-ray observations from two other NASA Great Observatories, the Spitzer Space Telescope and Chandra X-ray Observatory. The vast number of galaxies in the Legacy Field image are also prime targets for future telescopes. “This will really set the stage for NASA’s planned Wide Field Infrared Survey Telescope (WFIRST),” Illingworth said. “The Legacy Field is a path- finder for WFIRST, which will cap- ture an image that is 100 times larger than a typical Hubble photo. In just three weeks’ worth of obser- vations by WFIRST, astronomers will be able to assemble a field that is much deeper and more than twice as large as the Hubble Legacy Field.” In addition, NASA’s upcoming James Webb Space Telescope will allow as- tronomers to push much deeper into the legacy field to reveal how the in- fant galaxies actually grew. Webb’s infrared coverage will go beyond the limits of Hubble and Spitzer to help astronomers identify the first galaxies in the universe. !